1. Field of the Invention
The present invention relates to a signal processing method by maximum likelihood decoding and also relates to a method for rating an information recording medium by the maximum likelihood decoding.
2. Description of the Related Art
Recently, as the densities of optical disk media have been further increased, the shortest length of recording marks has come even closer to the limit of optical resolutions. As a result, intersymbol interference is occurring increasingly often and the SNR (signal to noise ratio) is decreasing even more significantly. To overcome these problems, it has become more and more common to adopt PRML (partial response maximum likelihood) as a signal processing method.
The PRML method is a combination of the partial response (PR) technique and the maximum likelihood (ML) technique, and is a method for selecting a most likely signal sequence based on a read signal waveform on the supposition that a known intersymbol interference should occur. Thus, it is known that decoding performance improves compared to the conventional level determination method (see, for example, Illustrated Blu-ray Disc Reader, Ohmsha, Ltd. (Non-Patent Document No. 1))
Meanwhile, as the level determination method has been gradually replaced by the PRML method as a mainstream signal processing method, some problems are arising about how to evaluate the read signal. Specifically, jitter has been used as an index for evaluating a read signal on the supposition that the signal processing technique is level determination, and therefore, may sometimes have no correlation with the decoding performance of the PRML method, which requires a different signal processing algorithm than that of the level determination. Thus, new indices that do have correlation with the decoding performance of the PRML method have been proposed (see, for example, Japanese Patent Application Laid-Open Publication No. 2003-141823 (Patent Document No. 1) and Japanese Patent Application Laid-Open Publication No. 2004-213862 (Patent Document No. 2)).
On the other hand, a new index that would contribute to detecting edge shifting between marks and spaces, which are very important factors that determine the storage quality of an optical disk medium, has also been proposed lately (see, for example, Japanese Patent Application Laid-Open Publication No. 2004-335079 (Patent Document No. 3)). As long as the PRML method is adopted, this index should also comply with the PRML rule and should also have correlation with the decoding performance of the PRML method. In addition, this index also needs to represent quantitatively the magnitude and direction of edge shifting on a pattern-by-pattern basis.
As described above, as the densities of optical disk media have been further increased, the intersymbol interference and SNR will have more and more serious consequences. Non-Patent Document No. 1 discloses that to maintain a system margin, the PRML methods need to be changed into a higher-order one. For example, if a 12 cm optical disk medium has a storage capacity of 25 GB per recording layer, the system margin can be maintained by adopting PR 1221 ML method. However, if the storage capacity per recording layer is 33.3 GB, then PR 12221 ML method should be adopted. It is expected that as the densities of optical disk media are further rising, the order of the PRML method to adopt should continue to get higher and higher in this manner.
An exemplary method for adjusting write parameters so as to generate the best read signal for maximum likelihood decoding is disclosed in Japanese Patent Application Laid-Open Publication No. 2004-335079 (Patent Document No. 3).
However, if such a high-order PRML method that would achieve high performance during a read operation were adopted and if adjustments were made during writing by that PRML method (particularly if the edge positions of recording marks are adjusted), then the write operation could not be performed with the write performance of the given recording medium taken full advantage of (e.g., under such a writing condition as to maximize the SNR) depending on the storage density of the medium, thus causing a decrease in the margin of the overall system. For example, if the PR 12221 ML method were adopted as the read signal processing method and if the best write adjustments were made for a PR 12221 ML read system, then 2T and 3T marks (where T is a channel width, or one period of a channel clock signal) as short recording marks would become too small to satisfy the best writing condition for the medium. As a result, the performance of repetitive writing might degrade too much and the deterioration with time might be noticeable too easily.
In addition, the read signals representing 2T, 3T and other short marks might have insufficient amplitudes, thus possibly causing deterioration in read performance due to a decrease in SNR or threatening the stability of a PLL circuit for detecting a sync signal from the read signal. That is to say, writing could be performed under such writing conditions as to decrease the read and write margins.
Also, if a higher-order PRML method is adopted to cope with the intersymbol interference that would be produced as the storage density is further increased (for example, PR 12221 ML is a higher order one than PR 1221 ML), the read signal waveform could be recognized with a waveform pattern of a longer interval in order to increase the resolution to determine the amplitude level of the read signal waveform and to identify the read signal even under the influence of the intersymbol interference. That is why confusing read signal patterns in the PR 12221 ML include patterns with multiple edges between marks and spaces. Naturally, these patterns are very useful in evaluating the read performance of the PRML method. If the pattern to evaluate included multiple edges, however, then it would be difficult to detect edge shifting between marks and spaces, which are very important factors that would determine the storage quality of an optical disk medium. In that case, the optical disk medium sometimes could not be rated appropriately.
An object of the present invention is to provide a good signal processing method and information recording medium evaluating method for such a system that adopts a high-order PRML method that would achieve high read performance. Another object of the present invention is to provide a method for adjusting writing conditions so as to maximize the system's margins.